The invention is based on a hydraulic dual-circuit brake system with an anti-skid system (ABS) and traction control (ASR) for motor vehicles.
In a known dual-circuit brake system of this type, with front-axle/rear-axle brake circuit distribution (German Patent 38 00 854 A1; U.S. Pat. No. 4,846,532), the brake fluid reservoir is embodied by the pressure chamber of a vacuum cell, which in traction control can be connected by means of a switching valve to the inlet of the pump element of the return pump assigned to the brake circuit for the driven wheels of the rear axle. The vacuum cell is divided by a diaphragm into two chamber sections. The diaphragm is connected to a pressure piston that is axially displaceable in the pressure chamber counter to a restoring spring and as a result produces a brake fluid pressure. To fill the pressure chamber with brake fluid, a vacuum is generated in the vacuum cell, so that the pressure piston, under the force of the restoring spring, makes the largest possible volume available in the pressure chamber; via the switch valve, this volume is filled with brake fluid from the master brake cylinder or from a separate brake fluid tank. In traction control, the outer chamber segment of the vacuum cell is vented. The negative pressure remaining in the other chamber segment effects a return of the diaphragm, which carries the pressure piston with it counter to the force of the restoring spring and thus generates a brake fluid pressure in the pressure chamber. Via the reversed switch valve, the brake fluid is fed at sufficient charge pressure into the pump element of the return pump. Simultaneously, the reversing valve in the connecting line between the master brake cylinder and the brake circuit of the driven wheels is reversed, so that that brake circuit is disconnected from the master brake cylinder.
During the entire traction control process, the vacuum cell generates a charge pressure that varies slightly and is adequate to supply the return pump with brake fluid. The return pump thus generates a permanently high brake supply pressure, which is above the maximum possible brake pressure of the wheel brake cylinders. The brake pressure necessary for braking a spinning driven wheel is established by constant switching of the control valve, assigned to this driven wheel, between a pressure buildup position and a pressure holding or pressure reduction position in the wheel brake cylinder of the spinning driven wheel.
If the brake pedal is actuated during traction control, then if no other provision is made a brake pressure buildup in the wheel brake cylinders of the wheels of the driven axle is not possible, while such a brake pressure is built up in the wheel brake cylinders of the wheels of the non-driven axle. This unilateral buildup of brake pressure leads to unstable road handling situations, which must be avoided. One such particular provision to avoid these unstable road- handling situations is that both the reversing valve and the switch valve are restored upon brake pedal actuation, so that on the one hand the communication between the master brake cylinder and the brake circuit of the driven wheels is re-established, and on the other the brake fluid reservoir is again disconnected from the brake circuit of the driven wheels. An electric signal transducer that recognizes the actuation of the brake pedal is necessary for this purpose. A brake light switch is typically used as such a signal transducer. However, if it should fail, the aforementioned disadvantage of unstable driving conditions arises.